In the oil and gas industry, the hydraulic fracturing is the main method used for increasing the productive capacity of a well through creation or expansion of channels from a wellbore to oil-bearing formations. This operation is generally accomplished by feeding hydraulically a fracturing fluid into a well which intersects subsurface rock. The fluid is injected into the rock beds at a high pressure sufficient to make a tension crack in the rock and to increase, as a result, the area of contact with the reservoir. Cracks occur in the rock or in the rock beds, and they form or expand one or more fractures, which usually results in increased production of oil from oil-bearing formations. A similar procedure is used for stimulating the production of gas from gas fields or the production of steam from geothermal sources. Ceramic or sand particles (proppant) are also injected into the well so that the well could be kept opened after the pressure has been relieved and the rock beds have closed. In situations where hydraulic fracturing is applied to carbonate-type rock, different acid systems are used for etching the outside surfaces of the fracture and for keeping them opened.
The post-fracturing productive capacity of the well depends on many factors, including the reservoir penetrability, porosity and pressure, as well as the properties of the fluid injected, etc. One of the most important factors is the fracture closure pressure. The fracture closure pressure is defined as the fluid pressure at which the existing fracture closes as a whole. The closure pressure forms the basis of the entire fracture analysis and is also used for proppant selection.
Various tests have been developed for determination of the fracture closure pressure, e.g. the injection/withdrawal test which determines the closure from different pressure decay rates (before and after the closure) during the fluid withdrawal to the surface at a constant flow rate; also, the pressure decay analysis which is based on identification of specimens and on calculations of the special time function (Nolte's G-plot); also, the post-closure analysis which is based on back calculations of the time to closure, calculated from the reservoir performance in case of a linear or transient inflow to the fracture. The introduction to these methods can be found in ‘Fracture Evaluation Using Pressure Diagnostics’, Chapter 9 of ‘Reservoir Stimulation’ published by John Wiley & Sons Ltd, 2000. This test is not commonly used under field conditions due to the inconvenience of installing a withdrawal pipeline maintaining a constant withdrawal rate.